By the year 1991, the particulate emission standards set by the Environmental Protection Agency (EPA) will require all urban buses to emit less than 0.1 gm/hp-hr of particulate matter. Further, by the year 1994, these standards will apply to all heavy duty trucks as well. Particulates are defined by EPA as any matter in the exhaust of an internal combustion engine, other than condensed water, which is capable of being collected by a standard filter after dilution with ambient air at a temperature of 125.degree. F. Included in this definition are, agglomerated carbon particles, absorbed hydrocarbons, including known carcinogens, and sulfates.
These particulates are very small in size, with a mass median diameter of 0.1-1.0 micrometers, and are extremely light weight. During the life of the typical vehicle, approximately 20 cubic feet of particulate matter which must be trapped will be emitted per 100,000 miles of engine operation. This amounts to slightly less than 100 lbs. of particulate matter. Obviously this particulate matter cannot be stored within the vehicle because one pound of particulate occupies a volume of approximately 350 cubic inches. Therefore, in order to meet these rigorous standards, there is a need for a filtration system which will efficiently, economically, and reliably remove these particulates from the exhaust emission of these vehicles.
A number of filtration systems have been developed over the years which employ a honeycomb or similar ceramic monolithic structure which is capable of trapping the small, light weight particulates as the exhaust gas is restricted to flow through such structure. However, after a predetermined amount of particulate matter is trapped by the filtration system or a predetermined period of time has lapsed, the ceramic structure must be regenerated, that is the particulate matter trapped within the structure must be oxidized. This is carried out in a variety of ways all of which are designed to significantly raise the temperature of the air or exhaust gas flowing through the ceramic structure as well as the ceramic structure itself.
One such regeneration system is disclosed in U.S. Pat. No. 4,404,795 issued to Oishi et al. The system includes a ceramic filter element for trapping particulate matter contained in the exhaust gas as such gas is passed therethrough. Once a predetermined amount of particulate matter has been trapped or a predetermined time period has lapsed, the filter is regenerated by way of electrically heated coils and ambient air, which is provided by way of an electric air feeding pump. The air stream is mixed with the exhaust gas and passed over the coils where such air/exhaust mixture is heated and passed through the filter to oxidize the particulate matter trapped therein. In order to prevent the backflow of exhaust gas into the air feeding pump a check valve is provided for allowing the flow of air in only a single direction.
With temperatures reaching a level of approximately 600.degree. C. within the aftertreatment system during a regeneration phase, and exhaust gas back pressures reaching a value of 3.0 psi during normal operation, it is imperative that the check valve employed to prevent the backflow of exhaust gas be resistant to these extraneous effects while also being resilient enough to allow air from an air feeding pump to flow therethrough when desired.
A similar exhaust gas aftertreatment system is disclosed in U.S. Pat. No. 4,581,891 issued to Usui et al. In this aftertreatment system a diesel fueled burner is employed which is periodically ignited by way of an igniter in response to a sensed condition within the exhaust gas stream. Once ignited the burning aspirated fuel will raise the exhaust gas temperature to approximately 600.degree. C. order to regenerate the particulate filter. In order for the aspirated fuel to properly ignite, a proper air/fuel mixture must be attained within the combustion chamber of the burner. Air for forming the proper air/fuel mixture is supplied to the combustion chamber by way of an air pump. However, with the above mentioned aftertreatment system, the air pump is not protected from the extreme temperatures which may be generated within the aftertreatment system nor is there any effort made to prevent the emission of particulate matter through the air pump during normal operating conditions.
One example of a flexible check valve is disclosed in U.S. Pat. No. 615,751 issued to Sands. The check valve is formed of rubber which must be of a significant thickness in order to resist deformation due to pack pressure while being resilient enough to allow fluid flow in a desired direction. Consequently, in instances where the back pressure is significant, the check valve would necessarily include thick walls which may not be readily opened by the forward flowing fluid pressure. Likewise, if the wall thickness is not sufficient to withstand the back pressure exerted thereon, it will collapse and turn inside out rendering the valve inoperable.
In an attempt to overcome the shortcoming discussed above, a highly flexible check valve is disclosed in U.S. Pat. No. 3,422,844 issued to Grise. This check valve is employed to control the direction of fluid flow therethrough in hydraulic systems. The check valve is formed of a relatively soft and resilient material with a pair of stiffening ribs for aiding in the check valves resistance to back pressure. Further, when the check valve is subjected to a significant back pressure, the valve is collapsed against a metallic plate which diametrically spans the base portion of the check valve to maintain the valve in a closed position, and to prevent the valve from turning inside out. This allows the body of the check valve to be made relatively thin when compared to the check valve discussed above. When the resilient valve member is subjected to increased back pressure, the deformation of the tapered walls is accompanied by substantial compression and deformation of the stiffening ribs. Further, there is a significant angular displacement of the extending portion of the valve about the base portion forming a circular hinge. Over time, this continued pivotal movement between an open positive flow condition and a closed and compressed condition may result in the permanent deformation of the check valve wherein the valve member may not retain its original configuration and consequently may not properly close thereby allowing unwanted fluid to pass therethrough. Further, continued pivotal movement of the valve member about the circular hinge may eventually exceed the valve's elastic limit and fracture the valve member at the hinge point resulting in leakage about the valve member. Each of these instances would require the shutdown of the system in which the valve is employed in order to replace the valve member. Also, because the valve member is permitted to flex through such a great distance, it is necessary to provide a stop plate of a size which significantly impedes the flow of fluid through the valve.
Therefore, there is clearly a need for a check valve which may be formed of a relatively thin flexible material which is prevented from deforming beyond its elastic limit and which will reliably open and close when necessary. Further, such a need particularly exists in the environment of an exhaust gas aftertreatment system wherein it is critical to provide the proper amount of air in order to oxidize particulate matter and protect the air feed pump from high temperature exhaust gas. Also, the check valve must be capable of preventing the leakage of untreated exhaust gas through the air feed pump and into the atmosphere.